Air purged optical densitometer

An advanced gas purged optical densitometer that significantly decreases the likelihood of optical component contamination and unstable optical path reduction includes an optical emitter, an optical receiver, and an optical path purge device. The optical emitter emits a light beam along an optical axis, through the optical purge path device and a test fluid. The optical receiver receives the light beam after it traverses the optical path purge device and the test fluid, and supplies a signal representative of intensity of the received light beam. The optical path purge device is configured to prevent contamination of the optical emitter and optical receiver, and to maintain a substantially optical path length of the test fluid.